Enantiomers of 2&#39;-Fluoralkyl-6-nitroquipazine as Serotonin Transporter Positron Emission Tomography Imaging Agents and Antidepressant Therapeutics

ABSTRACT

Racemic mixtures and individual enantiomers of fluorine-18 or carbon-11 radiolabelled 2′-alkyl-6-nitroquipazine ligands are serotonin transporter (SERT) tracers for positron emission tomography (PET) imaging. The non-radioactive ligand forms possess therapeutic antidepressant in vitro and in vivo pharmacological binding profiles in rodent brain and cells expressing human serotonin transporter (hSERT). Twelve 2′-alkyl-6-nitroquipazine ligands potently bind in sub-nanomolar concentrations to the pre-synaptic SERT binding site where established antidepressant drugs bind and inhibit the re-uptake of the neurotransmitter serotonin (5-HT). In vivo tracer studies in rats as well as monkey PET scan trial have demonstrated the fluorine-18 and carbon-11 positron radionuclide labeled tracers perform as quantitative tracers of specific binding the SERT protein in live brain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending application Ser. No.11/796,227, filed Apr. 26, 2007; which claims the benefits of U.S.Provisional Application No. 60/795,602, filed Apr. 26, 2006, thedisclosure of which is hereby incorporated by reference in its entiretyincluding all figures, tables and drawings.

This invention was made with Government support under Grant No. R15NS39814-02 awarded by the NIH. The Government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Altered serotonin (5-HT) biogenic amine functions within the centralnervous system (CNS) are involved or have been implicated in a number ofmental health disorders and clinical therapies. The determination ofpre-synaptic serotonin transporter (SERT) populations in discretecerebral regions of living brain may serve as an indicator of the 5-HTsystem in health and disease. Regional cerebral SERT densitymeasurements with positron emission tomography (PET) imaging arevaluable assessments of 5-HT terminal integrity in living brain. SinceSERT densities vary as a function of cerebral region, (e.g.,high-midbrain, hypothalamus; modest-limbic system, hippocampus andfrontal cortex; and low-cerebellum reference region) and with disease(e.g., within the limbic system and neocortical region, among otherregions), clinical PET imaging studies demand reproducible measurementsof a range of SERT densities (low to high) across live brain regions.Potent SERT tracers (positron labeled radioligands) with appropriate invivo profiles (brain penetration, target-to-reference tissue ratios,target tissue specific binding to particular protein, among others) arerequired. These in vivo tracer performance parameters as a function ofevaluating CNS 5-HT integrity with cerebral PET imaging for haverecently been discussed [Hesse 2004].

Primate brain SERT PET tracer investigations [Elfving 2001, Frankle2004, Huang 2004, Huang 2002] have brought forward a refined SERT tracerhypothesis, which contends that candidate imaging tracers suitable forassessing low-high SERT densities are plausible, prospectively when thetracers possess the certain traits, for example: i) high SERT bindingaffinity, ii) reduced nonspecific (reference region; e.g., cerebellum)binding, and iii) extended kinetic imaging profiles as a function of theenhanced radioligand affinity, compensated for with longer-livedradionuclides (e.g., fluorine-18) that provide extended timeframes forPET scan data collections affording accurate estimates of in vivo SERTdensities. A need remains for new and effective image tracers. Theidentification of tracers possessing the aforementioned in vitro and invivo qualities solves this problem.

All patents, patent applications, provisional patent applications andpublications referred to or cited herein, are incorporated by referencein their entirety to the extent they are not inconsistent with theexplicit teachings of the specification.

SUMMARY OF THE INVENTION

The invention encompasses novel [¹¹C]2′-alkyloxy- and[¹⁸F]2′-fluoroalkyl-6-nitroquipazine radioligands, that are candidatePET tracers for efficacious imaging of SERT densities across regions ofliving primate brain. The discovered [¹¹C]alkyloxy- and[¹⁸F]fluoroalkyquipazine tracers, as either a racemic mixture orenantiomeric pure forms, are with distinct molecular chemical structuresand pharmacological potency for the SERT target protein, they possesssuperior rodent in vivo cerebral penetration and regional cerebralspecific binding localizations, potency and other qualities as shown inrat by multiple studies and in monkey, according to a PET imaging study.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows biodistribution (% ID/g)) of tracer [¹⁸F]]2 in rats (tailvein injection) by organ dissection, and decay corrected radioactivitycounting, as a function of time; n=3.

FIG. 2 shows regional rat brain uptake (% ID/g) of tracer [¹⁸F]]2 decaycorrected radioactivity in dissected brain regions at various times;n=3.

FIG. 3 shows regional rat brain tracer uptake of [¹⁸F]12 at 120 min posttracer injection in control (untreated, saline) rats, and with ratsco-injected with tracer plus SERT inhibitors Paroxetine or the MOMligand 13 (co-injection does of 2.5 mg/kg); n=3.

FIG. 4 shows regional rat brain SERT binding of tracer [¹⁸F]]2 at 1-6 hpost tracer injection, delineating specific binding within the SERT richhypothalamus, moderate SERT density regions (hippocampus and frontalcortex) and nonspecific binding within cerebellum. Specific binding wasdefined as the difference between total binding radioactiviy and bindingradioactivity in the presecenfo SERT antidepressant inhibitor Paroxetine(co-injection dose at 2.5 mg/Kg). For all points (n=3) error bars havebeen omitted for graphical clarity. The arrow (4 h) is a time period oflow cerebellum activity and high specific binding within rat cerebralactivity and high specific binding within rat cerebral regions ofinterest. The dashed line is the time (3 h) during the rat study thatcorrelates to a monkey PET scan sampling (3 h) performed with tracer[¹⁸F]]2, as detailed in FIG. 5.

FIG. 5 shows cerebral PET images of the accumulation of tracer [¹⁸F] 2in macaca mulatta monkey coronal sections acquired at 3 h post tracerinjection, depicted in heat scale format.

DETAILED DESCRIPTION OF THE INVENTION

The compounds of the subject invention are serotonin transporterpositron emmission tomography imaging agent and antidepressanttherapeutics which are enantiomers of 2′-fluoralkyl-6-nitroquipazine.The compounds of the subject invention are described generally inFormula 1.

wherein R is selected from the group consisting of CH₂OCH₃, CH₂OH,CH₂CH₂CH₂OCH₃, CH₂CH₂CH₂OH, CH₂CH₂CH₂F and CH₂OCH₂CH₂CH₂F. Allstereoisomers, both enantiomers and diastereomers, and mixtures thereof,are considered to fall within the scope of the subject invention.

TABLE 1 Previously established 6-Nitroquipazine tracers and ligands,with respective SERT competitive binding potency K_(i) values. K_(i)^(a) Cpd R₂’ R₃’ R₃ R₅ (nM) Tracer Reference 5 H H H H 0.16^(a, b) 6 H HH I 0.17^(a, c) [¹²³I] Jagust 1993 & 1995 7 H H H Br 0.13^(a, c) [⁷⁶Br]Lundkvist 1999 8 H H H F 0.25^(a, c) [¹⁸F] Karramkan 2002 9 H H H CH₃  [¹¹C] Sandell 2002 10 H H Pr-F H 0.31^(a, d) [¹⁸F] Lee 1999, Lee 2003 11H CH₃ H H 4.56^(a, b) 12 CH₃ H H H 0.08^(a, b) ^(a)Rat brain,[³H]paroxetine. K_(i) references: ^(b)Ref. Gerdes 2000; ^(c)Ref. Mathis1993. ^(d)Rat brain, [³H]citalopram.

A partial summary of the 6-ntiroquipazine molecular framework for thediscovery of new SERT PET radioligands within the molecular class as afunction of attempts to transform the ligands into useful imagingtracers is found in Table 1, with respective references. For most of theanalogs shown, many possess high SERT affinity (Ki<1 nM) and themajority of the analogs are those with changes at position number 5 (R₅in Table 1). None of these established 6-nitroquipazine tracers howeverserve as useful PET imaging agents. Therefore, from an initial leadcompound, ligand 12, a novel analog series has been defined to serve asPET imaging agents (Table 2). The Table 2 ligands have been modifiedwith radiolabelling, which has resulted in the discovery of the racemicand enantiomeric pure forms of the [¹⁸F]2′-fluoroalkyl-6-nitroquipazinetracers 1 and 2 and [¹¹C]2′-alkyloxy-6-nitroquipazine radioligand 13 asnovel SERT agents.

R₁ R₂ H CH₂OCH₃[¹¹C] H CH₂CH₂CH₂F[¹⁸F] H CH₂OCH₂CH₂CH₂F[¹⁸F]CH₂OCH₃[¹¹C] H CH₂CH₂CH₂F[¹⁸F] H CH₂OCH₂CH₂CH₂F[¹⁸F] H

Novel 2′-alkyl-6-nitroquipazines, as described within Table 2 analogs1-2 and 13-16, in various stereochemical forms have been prepared andpossess high SERT binding potency with competitive binding K_(i) valuesin sub-nanomolar concentration range. The 2′-alkyl side chain groups asregions for attachment of a fluorine-18 or a carbon-11 radionuclideafford positron emission tomography imaging agents for the serotonintransporter. The carbon-11 and fluorine-18 radiolabelled forms of theanalogs are found useful for in vivo imaging of SERT density withindiscrete cerebral regions. Additionally, the non-radioactive forms ofthe compounds can be useful as antidepressant therapeutics based ontheir SERT binding profiles.

TABLE 2 Prepared and characterized 2′-Alkyl-6- nitroquipazine analogs ofthis invention (ligands 1-2, 13-16) and the known lead agent 12.

Cpd R Name Reference 12 CH₃ (reference) Me Gerdes 2000 13 (±)-CH₂OCH₃MOM 13R (R)-CH₂OCH₃ R-MOM 13S (S)-CH₂OCH₃ S-MOM 14 (±)-CH₂OH HOM 15(±)-CH₂CH₂CH₂OCH₃ PrOM 16 (±)-CH₂CH₂CH₂OH PrOH 1 (±)-CH₂CH₂CH₂F PrOF 1R(R)-CH₂CH₂CH₂F R-PrOF 1S (S)-CH₂CH₂CH₂F S-PrOF 2 (±)-CH₂OCH₂CH₂CH₂FMePrOF 2R (R)-CH₂OCH₂CH₂CH₂F R-MePrOF 2S (S)-CH₂OCH₂CH₂CH₂F S-MePrOF^(a)The symbol * indicates a stereochemical center, where (±) is theracemic form, (R) is the R-configurational enantiomer and (S) is theS-configurationaal enantiomer.

The ligands of Table 2 were prepared according to the synthetic routesdelineated in Schemes 1-3. The route exemplified in Scheme 1 is general,where a variety of starting material amino acids, similar to that shownas (S)-17, can be employed for the production of the Table 2 ligands.The starting material (S)-17 and similar starting material reagents areprepared by established procedures [Bedurftig 2004, Bedurftig 2006,Naylor 1993].

To a 0° C. solution of alcohol (S)-17 (0.567 g, 2.75 mmol) in formicacid (88%, 8 mL) was added (drop-wise) acetic anhydride (2.33 mL, 24.7mmol). The reaction was stirred for 30 min at 0° C. then warmed toambient temperature. Following 1 h the reaction was diluted with ice andmade basic with 4 N NaOH. The aqueous mixture was further diluted withsaturated NaHCO₃ and extracted with CH₂Cl₂. The combined extracts weredried (K₂CO₃) and concentrated to give an orange-brown oil that waspurified by column chromatography to afford(S)-(−)-4-Benzyl-2-(hydroxymethyl)piperazine-1-carbaldehyde, (S)-18 as acolorless oil (0.521 g, 67%). ¹H NMR (400 MHz, CDCl₃): δ 2.05 (td,J=3.7, 11.7 Hz, 0.5H), 2.13 (td, J=3.7, 11.7 Hz, 0.5H), 2.28 (dd, J=4.0,11.7 Hz, 1H), 2.87 (m, 1H), 2.90-3.02 (m, 1H), 3.12 (td, J=4.0, 12.8 Hz,0.5H), 3.41-3.59 (m, 3.5H), 3.64-3.75 (m, 1H), 3.85 (m, 0.5H), 3.97 (dd,J=5.5, 11.4 Hz, 0.5H), 4.08 (dd, J=7.3, 11.4 Hz, 0.5H), 4.20 (bd, 0.5H),4.38 (m, 0.5H), 7.25-7.36 (m, 5H), 8.06 (s, 0.5H), 8.08 (s, 0.5H).

To a 0° C. solution of alcohol (S)-18 (0.502 g, 2.14 mmol) in dry DMF(20 mL) was added NaH (95%, 0.154 g, 6.43 mmol) in one portion. Afterstirring 5 min, iodomethane (0.319 g, 2.25 mmol) was added (drop-wise)and the mixture stirred for 20 min at 0° C. then at ambient temp for 1.5h. The excess NaH was destroyed by the careful addition of water and thesolution was diluted with 40 mL each of water and saturated NaHCO₃. Theaqueous mixture was extracted with ether and ethyl acetate. The combinedextracts were washed with brine, then dried (K₂CO₃) and concentrated togive a brown oil that was purified by column chromatography to afford(S)-(−)-4-Benzyl-2-(methoxymethyl)piperazine-1-carbaldehyde, (S)-19 asan almost colorless oil (0.360 g, 69%). ¹H NMR (400 MHz, CDCl₃): δ2.00-2.12 (m, 1.3H), 2.18 (dd, J=3.7, 11.7 Hz, 0.7H), 2.80-2.92 (m, 2H),2.93-3.02 (m, 1H), 3.29-3.38 (m, 3H), 3.41-3.76 (m, 5H), 4.16 (bd,0.7H), 4.60 (m, 0.3H), 7.25-7.35 (m, 5H, overlapped with CDCl₃), 8.04(s, 0.7H), 8.07 (s, 0.3H).

A solution of (S)-20 (0.320 g, 1.29 mmol) in THF (3 mL) and 4 M H₂SO₄ (9mL) was heated at 55° C. for 5 h. After cooling, the reaction contentswere poured into 20 mL of cold (−10° C.) 4 M NaOH and diluted with 20 mLof saturated NaHCO₃. The mixture was extracted with CH₂Cl₂ and thecombined extracts dried (K₂CO₃) and concentrated to provide(S)-(+)-1-Benzyl-3-(methoxymethyl)piperazine, (S)-20 as a pale oilysolid (0.274 g, 96%). This product was of adequate purity for thesubsequent transformations. ¹H NMR (400 MHz, CDCl₃): δ 1.85 (t, J=10.3Hz, 1H), 2.11 (td, J=3.3, 11.0 Hz, 1H), 2.35 (bs, 1H, NH), 2.74 (m, 2H),2.90 (m, 1H, td shape), 2.96-3.06 (m, 2H), 3.25-3.36 (m, 5H, OCH₃singlet present at 3.33), 3.50 (m, AB pattern, J=13.2 Hz, 2H), 7.22-7.33(m, 5H, overlapped with CDCl₃).

To a 0° C. solution of (S)-20 (0.273 g, 1.24 mmol) in dry ether (16 mL)under argon was added (dropwise) a solution of n-butyllithium in hexane(2.45 M, 1.24 mmol) producing a clear yellow solution. After stirring 20min a solution of 2-chloroquinoline (0.134 g, 0.821 mmol) in ether (3mL) was added (dropwise) and the solution allowed to stir for 10 min at0° C. then at ambient temperature for 16 h. The reaction contents werediluted with ether, washed with saturated NaHCO₃ and brine, then dried(K₂CO₃) and concentrated to give the crude material that was purified bycolumn chromatography to provide(S)-(−)-2-[4-Benzyl-2-(methoxymethyl)piperazin-1-yl]quinoline, (S)-21 asa thick, light yellow oil (0.275 g, 96%). ¹H NMR (400 MHz, CDCl₃): δ2.17-2.27 (m, 2H), 2.95 (bd, J=10.3 Hz, 1H), 3.11 (bd, J=11.7 Hz, 1H),3.21 (m, 1H), 3.33 (s, 3H), 3.49-3.63 (m, 3H), 3.87 (m, 1H), 4.45 (bd,1H), 4.57 (bs, 1H), 6.98 (d, J=9.2 Hz, 1H), 7.21 (m, 1H), 7.27 (m, 1H,overlapped with CDCl₃), 7.31-7.39 (m, 4H), 7.52 (m, 1H), 7.59 (d, J=8.1Hz, 1H), 7.68 (d, J=8.4 Hz, 1H), 7.87 (d, J=9.2 Hz).

To a 0° C. solution of methyl ether (S)-21 (0.119 g, 0.34 mmol) in drydichloroethane (8 mL) under argon was added 1-chloroethyl chloroformate(0.093 g, 0.65 mmol). After stirring at 0° C. for 10 min the flask washeated at reflux for 2.5 h. The solution was cooled slightly and thevolatile components were evaporated. The residue was dissolved inmethanol (10 mL), heated at 60-70° C. for 1 h, and then the solvent wasevaporated. The residue was dissolved in 15 mL 1 M HCl and washed withCH₂Cl₂. The aqueous phase was made basic with 4 M NaOH, diluted withsaturated NaHCO₃ and extracted with CH₂Cl₂. The combined extracts weredried (K₂CO₃) and concentrated to give(S)-(−)-2-[2-(Methoxymethyl)piperazin-1-yl]quinoline, (S)-22 as a palecolored oil (0.081 g, 92%). ¹H NMR (400 MHz, CDCl₃): δ 1.93 (bs, NH,1H), 2.87 (m, 1H), 2.98 (dd, J=4.0 Hz and 12.5 Hz, 1H), 3.11-3.20 (m,2H), 3.29-3.37 (m, 4H, overlapped singlet at 3.36, OCH₃), 3.48 (dd,J=4.8 and 9.2 Hz, 1H), 3.86 (m, 1H), 4.36 (bd, 1H), 4.57 (bm, 1H), 6.97(d, J=9.2 Hz, 1H), 7.21 (m, 1H), 7.52 (m, 1H), 7.59 (m, 1H), 7.68 (d,J=8.8 Hz, 1H), 7.88 (d, J=9.2 Hz, 1H).

To a 0° C. solution of (S)-22 (0.038 g, 0.15 mmol) in conc. H₂SO₄ (2 mL)was added HNO₃ (15.4 M, 0.038 mL, 0.59 mmol). The reaction was stirred17 min then quenched by transfer onto ice. The solution was basifiedwith 4 M NaOH then diluted with saturated NaHCO₃ (10 mL). The brightyellow aqueous mixture was extracted with CH₂Cl₂ and the combinedextracts dried (K₂CO₃) and concentrated to provide(S)-(−)-2-[2-(Methoxymethyl)piperazin-1-yl]-6-nitroquinoline, (S)-13 asa bright yellow-orange oily solid (0.040 g, 91%). ¹H NMR (400 MHz,CDCl₃): δ 1.77 (bs, 11-1), 2.87 (td, J=3.3, 11.7 Hz, 1H), 2.98 (dd,J=4.4, 12.5 Hz, 1H), 3.15-3.28 (m, 2H), 3.32 (m, 1H), 3.37 (s, 3H), 3.60(dd, J=5.5, 9.2, 1H), 3.85 (dd, J=7.7, 9.2 Hz, 1H), 4.50 (bm, 1H), 4.66(bm, 1H), 7.08 (d, J=9.2 Hz, 1H), 7.65 (d, J=9.2 Hz, 1H), 7.96 (d, J=9.2Hz, 1H), 8.29 (dd, J=2.6, 9.2 Hz, 1H), 8.54 (d, J=2.6 Hz, 1H); ¹³C NMR(100 MHz, CDCl₃): δ 45.9, 46.3, 50.9, 59.1, 70.2, 110.9, 121.0, 123.5,124.2, 127.0, 138.5, 141.7, 151.4, 158.5; HRMS (ESI-TOF) m/z (M+H)⁺calcd. for C₁₅H₁₉N₄O₃ 303.1457. found 303.1448; [α]_(D) ²⁵ −137.8 (c0.0023, CHCl₃).

In another example, the ligand 2 can be prepared according to the routeof Scheme 2, employing the starting material 13. The Scheme 2 routeaffords either racemic or enantiomerically pure forms of the ligandsdepending upon the stereochemistry of the starting material 13.

To a −78° C. stirring solution of 13 (0.143 g, 0.474 mmol) in dry CH₂Cl₂(75 mL) under argon was added (drop-wise) borontribromide solution (1 Min CH₂Cl₂, 2.37 mL, 2.37 mmol). The reaction was maintained at −78° C.for 3 h then warmed to ambient temperature and stirred 18 h. Thereaction was quenched with saturated NaHCO₃, transferred to a separationfunnel and shaken. The organic phase was separated and the aqueous layerextracted with CH₂Cl₂ and CHCl₃ with isopropyl alcohol. The combinedorganic phases were dried (K₂CO₃), and concentrated to give a cruderesidue. The crude residue was dissolved in about 20 mL of CH₂Cl₂ and tothis was added a solution of ditert-butyldicarbonate (0.124 g, 0.569mmol) in CH₂Cl₂ (5 mL). The solution was stirred for 5 min, concentratedand the residue purified by column chromatography to afford2-[4-(tert-Butoxycarbonyl)-2-(hydroxymethyl)piperazin-1-yl]-6-nitro-quinoline,24 as a bright yellow foam (0.129 g, 70%). ¹H NMR (400 MHz, CDCl₃): δ1.47 (s, 9H), 3.09-3.62 (bm, 4H), 3.66-3.96 (bm, 2H), 3.98-4.43 (bm,3H), 4.82 (bm, 1H), 7.07 (d, J=9.2 Hz, 1H), 7.61 (d, J=9.2 Hz, 1H), 7.97(d, J=9.2 Hz, 1H), 8.25 (dd, J=2.6, 9.2 Hz, 1H), 8.47 (d, J=2.6 Hz, 1H).

To a 0° C. solution of 24 (0.194 g, 0.50 mmol) in dry DMF (8 mL) underargon was added NaH (95%, 0.046 g, 1.92 mmol) in one portion. Allylbromide (0.302 g, 2.5 mmol) was added (drop-wise) to the mixture and thereaction warmed to ambient temperature. After stirring 1.5 h thereaction mixture was carefully added to a separatory funnel containingether (30 mL) and 16 mL each of water and saturated NaHCO₃. The aqueousphase was separated and the organic phase was washed with brine, dried(K₂CO₃) and concentrated to provide the crude material that was purifiedby column chromatography to afford2-[4-(tert-Butoxycarbonyl)-2-(allyloxymethyl)piperazine-1-yl]-6-nitroquinoline,25 as a bright yellow foam (0.192 g, 90%). ¹H NMR (400 MHz, CDCl₃): δ1.49 (s, 9H), 2.96-3.37 (bm, 3H), 3.49-3.68 (m, 2H), 3.93-4.78 (bm, 6H),5.04-5.31 (bm, 2H), 5.82 (bm, 1H), 7.08 (d, J=9.2 Hz, 1H), 7.63 (d,J=9.2 Hz, 1H), 7.95 (d, J=9.2 Hz, 1H), 8.25 (dd, J=2.6, 9.2 Hz, 1H),8.49 (d, J=2.6 Hz, 1H).

To 25 (0.075 g, 0.175 mmol) in dry THF (600 μL) under argon was added asolution of 9-BBN in THF (0.5 M, 950 μL, 0.475 mmol). The reaction washeated to 55° C. and maintained for 1 h. After cooling to 0° C. 0.6 mLof 1 M NaOH was added (drop-wise) followed by 0.6 mL of 30% H₂O₂. Themixture was stirred for 5 min. The mixture was poured into 20 mL ofsaturated NaHCO₃ and the aqueous phase extracted with CH₂Cl₂. Thecombined extracts were dried (K₂CO₃) and concentrated to give the crudematerial that was purified by column chromatography to afford2-[4-(tertButoxycarbonyl)-2-((3-hydroxypropoxy)methyl)-piperazine-1-yl]-6-nitroquinoline,26 as a bright yellow foam (0.057 g, 73%). ¹H NMR (400 MHz, CDCl₃): δ1.49 (s, 9H), 1.73 (bm, 2H), 2.98-3.37 (bm, 3H), 3.41-3.89 (bm, 7H),4.02-4.53 (bm, 3H), 4.71 (bm, 1H), 7.06 (d, J=9.2 Hz, 1H), 7.64 (d,J=9.2 Hz, 1H), 7.96 (d, J=9.2 Hz, 1H), 8.25 (dd, J=2.6, 9.2 Hz, 1H),8.49 (d, J=2.6 Hz, 1H).

To a 0° C. solution of alcohol 26 (0.095 g, 0.213 mmol) in dry CH₂Cl₂(1.2 mL) and pyridine (100 μL) was added p-toluenesulfonyl chloride(0.170 g, 0.895 mmol) in one portion. The reaction was sealed underargon and maintained in an ice bath until the bath warmed to ambienttemperature. After 24 h the reaction contents were partitioned betweensaturated NaHCO₃ (20 mL) and CH₂Cl₂ (10 mL). The organic phase wasseparated and the aqueous phase was extracted with CH₂Cl₂. The combinedextracts were dried (K₂CO₃) and concentrated to give the crude materialthat was purified by column chromatography to afford2-[4-(tert-Butoxycarbonyl)-2-((3-(4-methylbenzenesulfonate)propoxy)methyppiperazine-1-yl]-6-nitroquinoline,27 as a bright yellow foam (0.103 g, 80%). ¹H NMR (400 MHz, CDCl₃): δ1.48 (s, 9H), 1.79 (bm, 2H), 2.42 (s, 3H), 2.94-3.30 (bm, 3H), 3.44-3.60(bm, 4H), 4.02 (bm, 2H), 4.10-4.24 (m, 2H), 4.50 (bm, 1H), 4.65 (bm,1H), 7.07 (d, J=9.2 Hz, 1H), 7.30 (d, J=8.1 Hz, 2H), 7.63 (d, J=9.2 Hz,1H), 7.72 (d, J=8.1 Hz, 2H), 7.95 (d, J=9.2 Hz, 1H), 8.26 (dd, J=2.6,9.2 Hz, 1H), 8.50 (d, J=2.6 Hz, 1H).

To a solution of 27 (0.0238 g, 0.040 mmol) in dry THF (400 μL) was addeda solution of TBAF in THF (1 M, 0.06 mL, 0.06 mmol). The reaction wassealed under argon and heated at 55-60° C. for 3 h. After cooling, theTHF was evaporated and the residue was purified by column chromatographyto afford the N-Boc protected intermediate as a yellow film (0.0137 g,77%). To this intermediate (0.0217 g, 0.048 mmol) in a flask at 0° C.was added cold (−10° C.) conc. H₂SO₄ (3 mL) and the flask was swirled toclear the sides of material. After 5 min the contents were transferredonto ice, made basic with 4 M NaOH and further diluted with saturatedNaHCO₃. The yellow aqueous mixture was extracted with CH₂Cl₂ and thecombined extracts were dried (K₂CO₃) and concentrated to give the crudematerial that was purified by column chromatography to afford2-[2-((3-Fluoropropoxy)methyl)piperazine-1-yl]-6-nitroquinoline, 2 as ayellow film (0.013 g, 80%). ¹H NMR (400 MHz, CDCl₃): δ 1.80-2.00 (m, 3H,overlapped bs of NH and dp, J_(F-H)=26.0 Hz, FCH₂CH₂—, J_(H-H)=6.2 Hz),2.87 (m, 1H), 2.99 (m, 1H), 3.12-3.36 (m, 3H), 3.58 (t, J=6.2 Hz, 2H),3.68 (dd, J=5.5, 9.5 Hz, 1H), 3.88 (dd, J=7.3, 9.5 Hz, 1H), 4.44 (dt,J_(F-H)=47.2, FCH₂—, J_(H-H)=5.9 Hz, 2H, overlapped with a bs at 4.50,1H), 4.66 (bs, 1H), 7.08 (d, J=9.2 Hz, 1H), 7.63 (d, J=9.2, 1H), 7.95(d, J=9.2, 1H), 8.28 (dd, J=2.6, 9.2 Hz, 1H), 8.52 (d, J=2.6, 1H); ¹³CNMR (100 mHz, CDCl₃): δ 30.7 (J_(C-F)=19.8 Hz, FCH₂CH₂—), 41.3, 45.9,46.4, 50.0, 67.0 (J_(C-F)=6.1 Hz, FCH₂CH₂CH₂—), 68.5, 81.0(J_(C-F)=164.8 Hz, FCH₂—), 111.1, 121.1, 123.6, 124.2, 127.1, 138.4,141.8, 151.4, 158.7; HRMS (ESI-TOF) m/z (M+H)⁺ calcd. for C₁₇H₂₂N₄O₃F349.1676. found 349.1674.

In another example, the ligand 1 may be prepared according to the routeof Scheme 3, employing the ligand 15 as starting material. A syntheticroute similar to that described in Scheme 1 affords the requisitestarting material 15.

A solution of 15 (0.19 g, 0.58 mmol) in 88% formic acid (1.5 mL) wastreated with acetic anhydride (Ac₂O) (0.54 g, 0.5 mL, 5.2 mmol) and thereaction stirred for 30 min. The reaction was quenched by pouring ontoice, then brought to basic pH with 4 M NaOH, buffered to pH 10 with sat.NaHCO₃. The basic portion was extracted with CH₂Cl₂. The organicportions were combined, dried (K₂CO₃), and concentrated in vacuo to giveN-Formyl-3-(3-methoxypropyl)-4-(6-nitroquinolin-2-yl)piperazine, 28 as ayellow oil (0.19 g, 91%). NMR (CDCl₃, 400 MHz) δ 1.45-1.90 (m, 4H), 2.9(dt, J=4.2 and 12.6 Hz, 0.5H), 2.98 (dd, J=3.9 and 13.3, 0.5H),3.14-3.43 (m, 7.5H), 3.44-3.51 (m, 0.5H), 3.58 (bd, 0.5H), 3.70 (bd,0.5H), 4.45 (bt, 1H), 4.74 (bs, 0.5H), 4.91 (bs, 0.5H), 7.70 (dd, J=7.4and 9.1 Hz, 1H), 7.63 (d, J=9.1 Hz, 1H), 7.98 (d, J=9.4 Hz, 1H), 8.09(s, 0.5H), 8.2 (s, 0.5H), 8.28 (dd, J=2.6 and 9.1 Hz, 1H), 8.51 (d,J=2.6 Hz, 1H).

A solution of 28 (0.19 g, 0.53 mmol) in dry CH₂Cl₂ (25 mL) was cooled at−78° C. in a dry ice acetone bath. A solution of BBr₃ (2.6 mL, 1 M inCH₂Cl₂, 2.6 mmol, Aldrich) was added drop-wise under argon and stirredfor 2 h, and then the reaction was warmed to RT° C. and stirred for 2.5h. The reaction was quenched by the addition of sat. NaHCO₃ (20 mL), theorganic portion was separated and the aqueous was extracted with CH₂Cl₂.The organic portions were combined, dried (K₂CO₃), filtered andconcentrated in vacuo and the resultant crude material was purified bycolumn chromatography to affordN-Formyl-3-(3-hydroxypropyl)-4-(6-nitroquinolin-2-yl)piperazine, 29. ¹HNMR (CDCl₃, 400 MHz) δ 1.48-1.96 (m, 5H), 2.88-3.08 (m, 1H), 3.24-3.45(m, 1.5H), 3.50 (dd, J=3.9 and 13.6 Hz, 0.5H), 3.61 (bd, J=13.3 Hz,0.5H), 3.66-3.84 (m, 2.5H), 4.24-5.15 (bm, 3H), 7.11 (d, J=9.4 Hz, 1H),7.76 (bs, 1H), 8.04 (d, J=9.1 Hz, 1H), 8.12 (s, 0.5H), 8.26 (s, 0.5H),8.33 (dd, J=2.6 and 9.4 Hz, 1H), 8.57 (d, J=2.6 Hz, 1H).

A solution of 29 (0.081 g, 0.24 mmol) in dry CH₂Cl₂ (0.5 mL) and drypyridine (0.029 g, 0.37 mmol) was cooled at 0° C. To the solution wasadded tosyl chloride (0.062 g, 0.33 mmol). The solution was allowed tostir at 25° C. for 3 h. The crude reaction mixture was purified bycolumn chromatography to give3-(4-Formyl-1-(6-nitroquinolin-2-yl)piperazin-2-yl)propyl-4-methylben-zenesulfonate,30 as a yellow foam (0.060 g, 51% yield). ¹H NMR (CDCl₃, 400 MHz) δ1.70-2.02 (m, 8H), 2.44 (s, ArCH₃, 3H), 2.90 (dt, J=4.2 and 12.6, 1H),3.03 (dd, J=3.9 and 13.3 Hz), 3.20-3.60 (m, 10H), 3.73 (bd, 1H),4.30-4.70 (4m, 4H), 4.88 (bs, 1H), 5.10 (bs, 1H), 7.07 (dd, J=2.9 and9.1 Hz, 2H), 7.33 (d, J=7.8 Hz, 2H), 7.66 (dd, J=4.5 and 9.4 Hz, 2H),7.78 (d, J=8.1 Hz, 2H), 8.03 (d, J=9.4 Hz, 2H), 8.12 (s, 1H), 8.26 (s,1H), 8.31 (dd, J=2.6 and 9.4 Hz, 2H), 8.55 (d, J=2.6 Hz, 2H).

A solution of3-(4-formyl-1-(6-nitroquinolin-2-yl)piperazin-2-yl)propyl-4-methylbenzene-sulfonate30 (0.056 g, 0.11 mmol) in dry THF (0.30 mL) was treated with a solutionof TBAF (0.16 mL, 1 M in THF, 0.16 mmol) to give a dark orange solution.The reaction mixture was heated at 55° C. for 2 h, and then wasconcentrated in vacuo and diluted with CHCl₃. The crude material waspurified by column chromatography to giveN-Formyl-3-(3-fluoropropyl)-4-(6-nitroquinolin-2-yl)piperazine, 31 as ayellow foam (0.025 g, 64%). ¹H NMR (CDCl₃, 400 MHz) δ1.60-2.00 (m, 4H),2.90 (dt, J=3.6 and 12.3 Hz, 0.5H), 3.00 (dd, J=4.2 and 13.3 Hz, 0.5H),3.20-3.40 (2m, 2H), 3.44-3.62 (m, 1.5H), 3.72 (bd, 0.5H), 4.32-4.60 (m,3H), 4.83 (bs, 0.5H), 5.07 (bs, 0.5H), 7.07 (dd, J=3.2 and 9.4 Hz, 1H),7.69 (m, 1H), 8.03 (d, J=9.1 Hz, 1H), 8.10 (s, 0.5H), 8.25 (s, 0.5H),8.32 (dd, J=2.6 and 9.1 Hz, 1H), 8.56 (d, J=2.3 Hz, 1H).

A solution of 31 (0.025 g, 0.072 mmol) in THF (0.5 mL) was treated withH₂SO₄ (4 M, 0.25 mL) and heated at 60° C. for 1 hour, then the reactionwas quenched by pouring onto ice. The mixture was brought to a basic pHwith 4 M NaOH, and buffered to pH 10 with sat. NaHCO₃. The aqueous layerwas extracted with CH₂Cl₂, the organic portions were combined, dried(K₂CO₃) and concentrated in vacuo to give2-(2-(3-Fluoropropyl)piperazin-1-yl)-6-nitroquinoline, 1 as yellow oil(0.021 g, 90%). NMR (CDCl₃, 400 MHz) δ 1.56-2.10 (2m, 5H), 2.91 (dt,J=3.3 and 12.3 Hz, 1H), 3.06 (dd, J=3.9 and 12.3 Hz, 1H), 3.14-3.38 (3m,3H), 4.40-4.60 (2m, 2H, CH₂F), 4.81 (bs, 1H), 7.04 (d, J=9.4 Hz, 1H),7.64 (d, J=9.1 Hz, 1H), 7.99 (d, J=9.4 Hz, 1H), 8.31 (dd, J=2.6 and 9.4Hz, 1H), 8.54 (d, J=2.6 Hz, 1H).

Three examples of the radiolabelled forms of the invention ligands aredescribed. These include carbon-11 and fluorine-18 radionuclideincorporations to afford the PET imaging tracers [¹¹C]13, ['¹⁸F]2S and[¹⁸F]1.

The first example is the transformation of compound 24 (Scheme 2) to theradiolabelled MOM ligand [¹¹C]13 (Table 2) which is accomplished withthe following protocol. Carbon-11 CO₂ was produced with a CTI RDS 111cyclotron by the ¹⁴N(p,α)¹¹C reaction with 1% O₂/N₂ and subsequentlytrapped on carbospheres at room temperature using an establishedprocedure [Jacobsen 1999]. The trap was heated to release the [¹¹C]CO₂which was converted to [¹¹C]CH₃I by the established method of Langstrom[Mock 1995]. A solution of 24 (2 mg) in DMF (0.25 mL) was treated withsodium hydride (3 mg) at room temperature (3 mL borosilicate V-vial).The vial was sealed, cooled to −5° C., and the gaseous [¹¹C]CH₃I waspassed through the solution. The mixture was heated for 2-3 min (100°C.) and then cooled to room temperature. The reaction was quenched withthe addition of ethanol (0.15 mL), diluted with water (55 mL) and thecrude 4′-amine-t-Boc ether intermediate (not shown) was isolated on aC-18 Sep-Pak cartridge. The cartridge was eluted with dichloromethane (2mL). Dilution of the cartridge eluant with TFA:dicholoromethane (1:9)was followed by concentration of the solution in vacuo at 100° C. (5min) to provide a crude residue of [¹¹C]13. Purification of the residuewith reversed phase semi-preparative HPLC (Activon GoldPak, Excil ODS-B10 μm, 250×10 mm; methanol:water:triethylamine, 1.86:1.0:0.0006)monitored with UV (254 nm) and radioactivity detection provided the pure(>95%) tracer [¹¹C]13. Routinely (n=4), [¹¹C]12 was afforded over atotal time of 45-60 min, in a decay corrected, end of bombardment yieldof 9-16%.

Another example of radiolabelling includes the formation of(S)-[¹⁸F]MePrOF tracer [¹⁸F]2S of the invention (Table 2). Radioactivefluoride ion was generated by irradiating oxygen-18 enriched water(>94%) in a silver coated target chamber with a 10 MeV proton beam of aCTI RDS-111 cyclotron. Following irradiation, the water (containingAg[¹⁸F]F, 175 μL) and dry acetonitrile (300 μL) were added to 2 μL oftetra-nbutyl-ammonium hydroxide (TBA-OH) in a siliconized vacutainer.The water was removed through azeotropic evaporation of thewater/acetonitrile mixture at 100° C. (3 cycles) to afford [¹⁸F]TBAFin >90% radiochemical yield. The [¹⁸F]TBAF was then transferred to areaction vial containing tosylate 27 (Scheme 2). The vial was sealed andheated at 100° C. for 10 min. The crude mixture was pushed through asilica gel cartridge with acetonitrile and evaporation of the solvent at100° C. provided the crude labeled material in >50% decay correctedradiochemical yield. The crude material was treated with conc. H₂SO₄ for10 min at 20° C. to remove the t-Boc protecting group. After dilutingwith 0.1 N NaOH, the solution was loaded onto a C-18 Sep-Pak cartridgeand rinsed with water. The crude radiotracer (S)-[¹⁸F]2 was eluted byflushing the cartridge with methanol. The methanol solution was dilutedwith water (1 mL) and the mixture was purified by semi-preparative HPLC(radioactivity detection). The product HPLC fraction was diluted withwater and loaded onto a C-18 Sep-Pak cartridge and then eluted from thecartridge with ethanol. The purified tracer is routinely (n=9) obtainedin a 15-30% decay corrected radiochemical yield in a completedradiochemical synthesis and purification time of approximately 90minutes.

Another example of radiolabelling includes the formation of [¹⁸F]PrOFtracer [¹⁸F]1 of the invention (Table 2) employing radiofluorinationconditions similar to those used for the formation of [¹⁸F]2S. The[¹⁸F⁻]F⁻ displacement of tosylate 30 utilizing [¹⁸F]TBAF in acetonitrileat 100° C., followed by treatment of the crude radiolabelled materialwith 4 N H₂SO₄ at 100° C., aqueous NaOH and then a C-18 Sep-Pakcartridge with methanol elution afforded [¹⁸F]1. Purification of [¹⁸F]1by semi-preparative HPLC (65:35:0.2 MeOH:H₂O:Et₃N) afforded pure [¹⁸F]1in a 8-11% end of bombardment, decay corrected radiochemical yield(n=3).

The compounds of the invention are potent (sub-nanomolar concentration)pharmacological inhibitor binding ligands of the serotonin transporter.Examples of the in vitro inhibition competitive pharmacological bindingSERT potencies are shown in Table 3, where the binding constant K_(i)values are reported as mean±s.e.m., n=3 or greater. The bindingconstants were determined using an established competition assay (ratfrontal cortical SERT, rSERT; [³H]Paroxetine, 3 h incubation time at 20°C., nonspecific binding determined as the difference in the absence andpresence of saturating levels of non-radioactive 6-nitroquipazine)[Gerdes 2000] which is a modified method of Habert [Habert 1985]. Themore potent enantiomeric stereochemical isomers are those with the(R)-configuration. The ligands of the invention are considered to haveantidepressant qualities since they effectively compete with theradiolabelled form of the known antidepressant Paroxetine([³H]Paroxetine) at the serotonin transporter [Tatsumi 1997, Hyttel1994].

TABLE 3 Examples of pharmacological binding of the 2′-alkyl-6-nitroquip-azine analogs of the invention relative to the established lead agent12.

K_(i) Cpd R Name (nM)^(b) Reference 12 CH₃ (reference) Me 0.081 ± 61  Gerdes 2000 13 (±)-CH₂OCH₃ MOM 0.42 ± 0.01 13R (R)-CH₂OCH₃ R-MOM 0.25 ±0.03 13S (S)-CH₂OCH₃ S-MOM 1.52 ± 0.10 14 (±)-CH₂OH HOM 0.68 ± 0.04 2(±)-CH₂OCH₂CH₂CH₂F MePrOF 0.28 ± 0.09 2R (R)-CH₂OCH₂CH₂CH₂F R-MePrOF0.25 ± 0.08 2S (S)-CH₂OCH₂CH₂CH₂F S-MePrOF 1.56 ± 0.88 ^(a)The symbol *indicates a stereochemical center, where (±) is the racemic forms, (R)is the R-configurational enantiomer and (S) is the S-configurationalenantiomer. ^(b)Competitive binding, rat brain cortex homogenate,[³H]paroxetine, 20° C., 3 h; n = 3.

In other pharmacological experiments, as shown in Table 4, the analogsof the invention were found to competitively inhibit tritiated serotonin([³H]5-HT) uptake into HEK-293 cells transfected with human serotonintransporter (hSERT) using an established procedure [Henry 2003, Henry2006] that is carried out at 20° C. and for 3 h incubation time. Theanalog inhibition K_(i) values occur with sub-nanomolar concentrations,where it is found that the (R)-configurational stereoisomers are withgreater potency than the opposing (S)-enantiomeric forms. The ligands ofthe invention are considered to have antidepressant qualities since theyeffectively inhibit the uptake of the radiolabelled form of theneurotransmitter 5-HT by the human serotonin transporter [Hyttel 1994].

TABLE 4 Examples of pharmacological inhibition of 5-HT up- take byanalogs of this invention.

K_(i) Cpd R Name (nM)^(b) 13 (±)-CH₂OCH₃ MOM 0.47 13R (R)-CH₂OCH₃ R-MOM0.15 13S (S)-CH₂OCH₃ S-MOM 1.66 ^(a)The symbol * indicates astereochemical center, where (±) is the racemic forms, (R) is theR-enantiomer and (S) is the S-enantiomer. ^(b)Competitive binding, hSERTHEK-293 cells, [³H]5-HT, 20° C., 3 h.

Example 1 In vivo Tracer Studies in Rats

Examples of the biodistributions of the invention include studies withtracer [¹⁸F]2 in rats as shown in FIGS. 1-4, which are consideredrepresentative of in vivo performance as an imaging agent for SERT[Huang 2005, Biegon 1993]. The biodistribution protocols followedestablished sacrifice, dissect and count methods [Biegon 1993] underseveral conditions, utilizing male Sprague-Dawley rats, lateral tailvein injection of tracer with three rat subjects per group and whereaverage decay corrected percent injected dose activity values per gramtissue (% ID/g)±s.e.m. error bars are shown. FIG. 1 demonstrates asignificant percentage of the injected tracer dose penetrates the brain(>1%), relative to the other tissues examined. FIG. 2 further delineatesa high partition of the tracer into various brain regions, where thehypothalamus, frontal cortex and hippocampus regions are associated withthe cerebral limbic system and are found to be with moderate SERTprotein densities whereas the cerebellum is with low SERT proteindensity and considered a reference region. The later 240 minute timepoints of FIG. 2 describe favorable respective relative ratios of traceractivity within tissues, including hypothalamus, frontal cortex orhippocampus vs. the low SERT density cerebellum region. FIG. 3delineates the blocking experiments of tracer [¹⁸F]2 in various cerebraltissues at 120 minutes post tracer injection, in the absence andpresence (co-injection with tracer at 2.5 mg/Kg) of the non-radioactivepotent antidepressant Paroxetine or ligand 13, thereby delineating thatthe tracer is specifically bound to the SERT protein within the cerebralregions. FIG. 4 demonstrates the SERT specific binding of the tracer indifferent cerebral regions over time, where specific activity is definedas the radioactivity difference between the absence and presence ofnon-radioactive antidepressant Paroxetine [Biegon 1993, Jagust 1993,Jagust 1995]. The FIG. 4 later time points of 4 and 6 h delineate thatthe specific tracer binding tissue ratios of hypothalamus, frontalcortex or hippocampus vs. cerebellum are high, thus demonstratingefficacy of the tracer for quantitative determinations of SERT densitiesin cerebral regions of interest useful for PET imaging of SERT in livebrain [Huang 2005, 2004, 2002].

Example 2 Monkey PET Scan with Tracer [¹⁸F]2

To associate tracer [¹⁸F]2 brain penetration and regional cerebrallocalization of in rat (e.g., Graph 4) to non-human primate brain andcerebral regions of interest with various known SERT densities, monkey(macaca mulatta) PET scans were performed. A portion of the resultsobtain are shown in FIG. 2. In brief, the monkey was anesthetized(ketamine, 15 mg/Kg, i.m. injection), intubated and placed on isofluraneanesthesia and then kept normothermic (heating blanket), hydrated(saphenous catheter), and pO₂ was monitored with an oximeter. Thesubject was placed in a stereotaxic frame, positioned in a Siemens-CTIECAT EXACT HR 47-slice PET imaging scanner in two-dimensional (2D)acquisition mode. Monkey cerebral slice images were acquired in thecoronal plane, after a 20 min transmission scan was obtained (using arotating ⁶⁸Ge source consisting of 3 rods of 2 mCi/rod) to correct forphoton attenuation. Emission data were collected according to the 3 hprotocol that began simultaneously with the injection of tracer [¹⁸F]2(i.v., 5 mCi; specific activity 3200 mCi/μmol). Coronal PET images weresampled over 0-3 h for the accumulation of [¹⁸F]2 providing theopportunity to compare activity found within regions of interest (ROIs)vs. the cerebellum reference region using the method of Logan [Logan1990]. Cerebral 2D ROIs were drawn for multiple ROIs (brain stem,thalamus, hypothalamus, hippocampus, putamen, frontal cortex,cerebellum, among others) with reference to a previously acquired monkeyMRI scan that together were correlated to an established monkey brainatlas. Representative coronal slice data sampled at 3 h are depicted inFIG. 2 (heat scale format), shown as panel A (SERT rich and moderatedensity, thalamus and hypothalamus, respectively) and panel B (SERTlow-moderate density region, frontal cortex). The comparative analysisof macaca mulatta PET scan data over time, per cerebral regions ofinterest relative to the cerebellum reference region, reveal highcerebral ROI:cerebellum ratios that correlate to the rat SERT specifictracer binding ratios established in FIG. 4.

It is understood that the foregoing examples are merely illustrative ofthe present invention. Certain modifications of the compounds and/ormethods employed may be made and still achieve the objectives of theinvention. Such modifications are contemplated as within the scope ofthe claimed invention.

REFERENCES

-   Bedurftig S, Wunsch B (2004) Chiral, nonracemic    (piperazin-2-yl)methanol derivatives with [sigma-receptor affinity.    Bioorg Med Chem 12:3299-3311.-   Bedurftig S, Wunsch B (2006) Synthesis and receptor binding studies    of 3-substituted piperazine derivatives. Eur J Med Chem 41:387-396.-   Biegon A, Mathis C A, Hanrahan S M, Jagust W J (1993)    [¹²⁵I]5-Iodo-6-nitroquipazine: a potent and selective ligand for the    5-hydroxytryptamine uptake complex. II. In vivo studies in rats.    Brain Res 619:236-46.-   Bishop J E, Mathis C A, Gerdes J M, Whitney J M, Eaton A M, Mailman    R B (1991) Synthesis and in vitro evaluation of    2,3-dimethoxy-5-(fluoroalkyl)-substituted benzamides: high affinity    ligands for CNS dopamine D₂ receptors. J Med Chem 34:1612-1624.-   Elfving B, Bjornholm B, Ebert B, Knudsen G M. (2001) Binding    characteristics of selective serotonin reuptake inhibitors with    relation to emission tomography studies. Synapse 41:203-11.-   Frankle G W, Huang Y, Hwang D-R, Talbot P S, Slifstein M, Van    Heertum, R, AbiDargham A, Laruelle M (2004) Comparative evaluation    of serotonin transporter radioligands ¹¹C-DASB and ¹¹C-McN 5652 in    healthy humans. J Nuc Med 45:682-694.-   Gerdes J M, DeFina S C, Wilson P A, Taylor S E (2000) Serotonin    transporter inhibitors: synthesis and binding potency of 2′-methyl-    and 3′-methyl-6-nitroquipazine. Bioorg Med Chem Lett 10:2643-2646.-   Habert E, Graham D, Tahraoui L, Claustre Y, Langer S Z (1985)    Characterization of [³H]paroxetine binding to rat cortical    membranes. Eur J Pharmacol 118:107-114.-   Henry K L, Adkins E M, Han Q, Blakely R D (2003) Serotonin and    cocaine-sensitive inactivation of human serotonin transporters by    methanethiosulfonates targeted to transmembrane domain I. J. Biol.    Chem. 278:37052-37063.-   Henry L K, Field J R, Adkins E M, Parnas M L, Vaughan R A, Zou M-F,    Newman A H, Blakely R D (2006) Tyr-95 and Ile-172 in transmembrane    segments 1 and 3 of the human serotonin transporters interact to    establish high affinity recognition of antidepressants. J. Biol.    Chem. 281:2012-2023.-   Hesse S, Barthel H, Schwarz J, Sabri O, Muller U (2004) Advances in    in vivo imaging of serotonergic neurons in neuropsychiatric    disorders. Neurosci Biobehavioral Rev 28:547-563.-   Huang Y, Bae S-A, Zhu Z, Gui N, Roth B L, Laruelle M (2005)    Fluorinated diaryl sulfides as serotonin transporter ligands:    synthesis, structure-activity relationship study, and in vivo    evaluation of fluorine-18-labeled compounds as PET imaging agents. J    Med Chem 48:2559-2570.-   Huang Y, Hwang D-R, Bae S-A, Sudo Y, Guo N, Zhu Z, Narendran R,    Laruelle M (2004) A new positron emission tomography imaging agents    for the serotonin transporter: synthesis, pharmacological    characterization and kinetic analysis of    [¹¹C]2-[2-(dimethylaminomethyl)phenylthio-5-fluoromethylphenylamine    ([¹¹C]AFM). Nuc Med Biot 31:543-556.-   Huang Y, Hwang D R, Narendran R, Sudo Y, Chatterjee R, Bae S A,    Mawlawi O, Kegeles L S, Wilson A A, Kung H F, Laruelle M. (2002)    Comparative evaluation in non-human primates of five PET    radiotracers for imaging the serotonin transporters: [¹¹C]McN 5652,    [¹¹C]ADAM, [¹¹C]DASB, [¹¹C]DAPA, and [¹¹C]AFM. J Cereb Blood Flow    Metab 22:1377-98.-   Hyttel J (1994) Pharmacological characterization of selective    serotonin reuptake inhibitors (SSRIs). Int. Clinical Psychopharmacol    (9 Suppl) 1:19-26.-   Jacobsen E J, Stelzer L S, TenBrink R E, Belonga K L, Carter D B, Im    H K, Im W B, Sethy V H, Tang A H, VonVoigtlander P F, Petke J D,    Zhong W-Z, Mickelson J W (1999) J Med Chem 42:1123.-   Jagust W J, Eberling J L, Roberts J A, Brennan K M, Hanrahan S M,    VanBrocklin H, Enas J D, Biegon A, Mathis C A (1993) In vivo imaging    of the 5-hydroxytryptamine re-uptake site in primate brain using    single photon emission computed tomography and    [¹²³I]5-iodo-6-nitroquipazine. Eur J Pharmacol 242:189-93.-   Jagust W J, Eberling E L, Biegon A, Taylor S E, VanBrocklin H F,    Jordan S, Hanrahan S M, Roberts J A, Brennan K M, Mathis C A (1995)    Iodine-123-5-iodo-6-nitroquipazine: SPECT radiotracer to image the    serotonin transporter. J Nuc Med 37:1207-14.-   Karramkam M, Dolle F, Valette H, Besret L, Bramoulle Y, Hinnen F,    Vaufrey F, Franklin C, Bourg S, Coulon C, Ottaviani M, Delaforge M,    Loc'h C, Bottlaender M, Crouzel C. (2002) Synthesis of a    fluorine-18-labelled derivative of 6-nitroquipazine, as a    radioligand for the in vivo serotonin transporter imaging with PET.    Bioorg Med Chem 10:2611-23.-   Lee B S, Chu S, Lee K C, Lee B-S, Chi D Y, Choe Y S, Kim S E, Song Y    S, Jin C (2003) Synthesis and binding affinities of 6-nitroquipazine    analogs for serotonin transporter: Part 3. A potential 5-HT    transporter imaging agent, 3-(3-[¹⁸F]-fluorpropyl)-6-nitroquipazine.    Biorg Med Chem 11:1-10.-   Lee B S, Lee K C, Chu S Y, Choy Y S, Kim B-T, Chi D Y (1999) A    potential 5-HT transporter imaging agent:    3-(3-[¹⁸F]fluoropropyl)-6-nitroquipazine. J Label Compd Radiopharm    42(suppl 1):54.-   Logan J, Fowler J, Volkow N D, et al. (1990) Graphical analysis of    reversible radioligands binding from time-activity measurements    applied to [N—¹¹ Cmethyl]-(−)-cocaine PET studies in human subjects.    J Cereb Blood Flow Metab 10:740-747.-   Lundkvist C, Loch C, Halldin C, Bottlaender M, Ottaviani M, Coulon    C, Fuseau C, Mathis C, Farde L, Maziere B. (1999) Characterization    of bromine-76-labelled 5-bromo-6-nitroquipazine for PET studies of    the serotonin transporter Nucl Med Biol 26:501-7.-   Mock B H, Vavrek M T, Mulholland G K (1995) Solid-phase reversible    trap for [¹¹C]carbon dioxide using carbon molecular sieves. Nucl Med    Biol 22:667-670.-   Naylor A, Judd D B, Lloyd J E, Scopes D I C, Hayes A G, Birch P    J (1993) A potent new class of k-receptor agonists: 4-substituted    1-(arylacetyl)-2-[(dialkylamino)methyl]piperazines. Med Chem    36:2075-2083.-   Sandell J, Halldin C, Sovago J, Chou Y—H, Gulyás B, Yu M, Emond P,    Någren K, Guilloteau D, Farde L (2002) PET examination of    [¹¹C]5-methyl-6-nitroquipazine, a radioligand for visualization of    the serotonin transporter. Nuc Med Biol 29:651-656.-   Tatsumi M, Groshan K, Blakely R D, Richelson E (1997)    Pharmacological profile of anti-depressants and related compounds at    human monoamine transporters. Eur J Pharmacol 340:249-258.

1. A compound having the following structure:

wherein R is selected from the group consisting of CH₂OCH₃, CH₂OH,CH₂CH₂CH₂OCH₃, CH₂CH₂CH₂OH, CH₂CH₂CH₂F and CH₂OCH₂CH₂CH₂F andenantiomers thereof.
 2. The compound of claim 1, wherein R is CH₂OCH₃.3. The compound of claim 1, wherein R is CH₂OH.
 4. The compound of claim1, wherein R is CH₂CH₂CH₂OCH₃.
 5. The compound of claim 1, wherein R isCH₂CH₂CH₂OH.
 6. The compound of claim 1, wherein R is CH₂CH₂CH₂F.
 7. Thecompound of claim 1, wherein R is CH₂OCH₂CH₂CH₂F.
 8. A pharmaceuticalcomposition comprising a compound having the following structure:

wherein R is selected from the group consisting of CH₂OCH₃, CH₂OH,CH₂CH₂CH₂OCH₃, CH₂CH₂CH₂OH, CH₂CH₂CH₂F and CH₂OCH₂CH₂CH₂F andenantiomers thereof; and a pharmaceutically acceptable carrier.
 9. Amethod of treating depression comprising administering a therapeuticallyeffective amount of a compound having the following structure:

wherein R is selected from the group consisting of CH₂OCH₃, CH₂OH,CH₂CH₂CH₂OCH₃, CH₂CH₂CH₂OH, CH₂CH₂CH₂F and CH₂OCH₂CH₂CH₂F andenantiomers thereof.
 10. The method of claim 9, wherein R is CH₂OCH₃.11. The method of claim 9, wherein R is CH₂OH.
 12. The method of claim9, wherein R is CH₂CH₂CH₂OCH₃.
 13. The method of claim 9, wherein R isCH₂CH₂CH₂OH.
 14. The method of claim 9, wherein R is CH₂CH₂CH₂F.
 15. Themethod of claim 9, wherein R is CH₂OCH₂CH₂CH₂F.